1. Field of Invention
The invention relates to a method for measuring electron diffraction of a sample by transmission. The invention further relates to a device for deflecting an electron beam incident on a sample and diffracted and undiffracted electron beams transmitted through the sample.
Electron diffraction is a well known method in the field of crystallography for obtaining structure information about crystals. In electron diffraction methods an electron beam is directed onto a sample diffracted by the atomic structure of the sample. Depending on the structure of the sample, like a crystal lattice structure, diffraction leads to reflections at distinct scattering angles, including Bragg reflections. The intensities of the diffracted reflections can be detected as a function of the scattering angle. Such measurement of diffracted intensities as a function of the scattering angle will be called a diffraction pattern within this application. From the measured intensities in the diffraction pattern information about the structure of the samples can be obtained. Electron diffraction can be measured by transmission or by reflection.
2. Description of Related Art
Due to a strong interaction between the scattered electrons and the sample material electron diffraction for crystallography is especially useful for determining the atomic structure of small crystalline volumes. The strong interaction of electrons with the sample material in electron diffraction measurements causes the electrons to scatter multiple times within the sample. This dynamical scattering leads to difficulties in the analysis of measured intensity data for structure determination. Also, by detecting the intensities of the diffracted beams, information about the phase of the diffracted electron beams is lost. This phase information is important for reconstructing the sample structure from the diffraction pattern. Therefore, for obtaining structure information from an intensity diffraction pattern, phase information must be reconstructed during the data analysis. Structure determination from x-ray-scattering methods usually makes use of the kinematic approach for solving the phase problem and analyzing measured intensity data. The kinematic theory neglects dynamical effects due to multiple scattering in the sample and is widely applicable for x-ray scattering methods. In electron scattering methods, because of the multiple scattering, the kinematic theory is not generally directly applicable. For reducing dynamical scattering effects due to multiple scattering, very thin samples are usually preferred.
Other methods for reducing the dynamic effects on electron diffraction patterns and thus enabling the application of the kinematic theory for data analysis include, for example, precession electron diffraction (PED).
In this approach described by R. Vincent and P. A. Midgley “Double conical beam-rocking system for measurement of integrated electron diffraction intensities” Ultramicroscopy 53 (1994) 271-282, an electron beam illuminating a sample is conically scanned around the optical axis. Below the sample, the transmitted and diffracted beam is de-scanned for compensating the deflection of the incident beam from the optical axis. As a consequence, diffracted intensities integrated over the various angles of incidence which the incident electron beam assumes during scanning, are detected in the diffraction pattern. In the method described by Vincent and Midgley, a complete de-scanning of the beam below the specimen is essential for an efficient reduction of dynamical effects and measurement of the integrated Bragg intensities relative to a diffuse scattering background. Integrating the measured intensities over the various angles of incidence, however, leads to a loss of structural information.
On the other hand, it is known that disc-shaped diffraction patterns of distinct reflections produced from convergent beam electron diffraction (CBED) can be used for solving the phase problem even if the electrons have scattered multiple times (C. T. Koch and J. C. H. Spence in J. Phys. A: Math. Gen. 36 (2003) 803-816 “A useful expansion of the exponential of the sum of two non-commuting matrices, one of which is diagonal”). In CBED methods the sample is illuminated by a convergent beam having a specific convergence angle. The convergent beam therefore includes beam fractions hitting the sample under various angles of incidence up to the convergence angle. The various angles of incidence included in the convergent incident beam lead to a shift of the diffraction pattern for each angle of incidence. Thus each reflection in the CBED diffraction pattern has the shape of a disk. The larger the convergence angle, the larger the diameter of the reflections in the diffraction pattern. Therefore, the angular range, which can be reached with convergent beam electron diffraction is limited by half the distance between adjacent Bragg peaks in the diffraction pattern. If the angular range exceeds this limit, adjacent reflections overlap in the diffraction pattern and cannot be resolved any more. Therefore, the structural information which can be obtained with this method is limited.
All references cited herein are incorporated herein by reference in their entireties.